Conventional electrophotographic color apparatus forms two or more single color images on a photoconductive image member and transfers those images in registration to a receiving sheet. The receiving sheet is generally a cut sheet which has been positioned on the periphery of a roller. The roller rotates in contact or at least in transfer relation with the photoconductive member to bring the sheet through such transfer relation once for each color image to form a multicolor image.
When cut sheets of varying size are to be received by a transfer roller, multiple sets of gripping means must be provided. For example, if vacuum holes are used, typically a single set of holes grips the leading edge of each sheet, but a different set is necessary for the trailing edge of each different size. Vacuum holes create problems in transfer, both electrostatic field and thermal transfer for the portion of the image over the holes. Leading and trailing edge holes actually used can be positioned in margins, if margins are acceptable, but other sets occur in image areas if varying sizes are to be accommodated.
In the photographic printing art it is known to use continuous webs of photographic paper to form prints of different size for later cutting. Thus, a single roll of paper can be used to form a variety of sizes of photographic prints. The necessity of rotating the receiving sheet through the transfer zone once for each color in electrophotography has always prohibited use of this feature in such apparatus.
Japanese Kokai 63-64065 laid-open Mar. 22, 1988 discloses a continuous web color electrophotographic apparatus in which a web is fed using a pair of feed rollers through transfer relation with the photoconductor. The feed rollers are then reversed first to feed the web back to its starting point and second for re-movement through the transfer position to receive a second image. Registration is maintained by lining up marks on the photoconductor and the web for each transfer.
Accurate registration using marks on the photoconductor and the web with feed rollers requires accurate sensing devices and accurate control which, to say the least, stretches the technology for highest quality work. Perhaps more significant for high quality work, no web tracking device can prevent some crosstrack misalignment and skew between images, each of which will impart misregistration to the ultimate multicolor images.